1. Field of the Invention
The present invention relates to a liquid crystal display device and a lens sheet used for the liquid crystal display device, and in particular, relates to a direct-view type liquid crystal display device used for office automation (OA) equipment such as word processors and notebook personal computers, various video-related equipment, game machines, TV receivers, and the like, and a lens sheet used for such a liquid crystal display device.
2. Description of Related Art
Conventionally, CRTs were often used as displays for personal computers, word processors, TV receivers, and the like. In recent years, with the demands for reduction in the size, thickness, and weight of these electronic apparatus, flat-panel displays have been increasingly used. Some types of flat-panel displays have been developed. Among these, liquid crystal display devices, which have advantages such as low power consumption, have found broad application.
A liquid crystal display device displays an image by applying an electric field to or passing a current through arbitrary display units of the display device to change light transmittance or reflectivity of the display units, utilizing electrooptic effects, including optical anisotropy (refractive index anisotropy), orientation property, flowability, and dielectric anisotropy, of liquid crystal molecules. Liquid crystal display devices are classified into direct-view type display devices permitting direct observation of an image displayed on the display devices and projection type display devices where an image is projected on a screen from the front or the back and the projected image is observed.
Direct-view type display devices have various display modes, such as a dynamic scattering mode, a twisted nematic mode, a super-twisted nematic mode, a polymer dispersion mode, a ferroelectric liquid crystal mode, a homeotropic mode, and a guest-host mode. As for driving of the direct-view type display devices, there have been developed driving schemes such as segment driving, simple matrix driving, and active matrix driving. Among these modes and driving schemes, the twisted nematic mode in the segment driving is often adopted when the number of display units is small, and the super-twisted nematic mode in the simple matrix driving is often adopted when the number of display units is large.
Liquid crystal display devices display information such as characters and graphics. In recent years, with the demands for increase in the capacity of display contents, a so-called dot matrix display style is often adopted, where minute display units are arranged in rows and columns for display of arbitrary information.
A direct-view type liquid crystal display device is constructed of a liquid crystal cell having an optical shutter function as a core in combination with a back light source for illumination from the back, a reflection prevention film for preventing the front surface from reflecting external light, and the like, as required.
There have been proposed techniques for minimizing the change in display quality with the direction in which a liquid crystal display device is viewed and widening the viewing angle that provides good display quality. These techniques are roughly classified into two: methods where the construction inside the liquid crystal display cell is improved and methods where the construction outside the liquid crystal display cell is improved. Examples of the former methods include: a method where liquid crystal molecules are modified; a method where the placement of polarizing elements, the orientation direction of liquid crystal molecules, and the like are optimized; a method where a plurality of films having birefringence are placed inside the liquid crystal display device; a method where minute concave and convex portions are formed in a substrate; and a method where the driving scheme is optimized. An example of the latter methods is a method where the liquid crystal display cell is combined with a lens, an element for controlling the light transmission direction, or the like.
As the method for widening the viewing angle by combining the liquid crystal display cell with a light diverging element such as a lens for controlling the light transmission direction placed on the viewer""s side of the liquid crystal display cell, the following methods are known. One is disclosed in Japanese Laid-Open Patent Publication No. 8-201796, where blurring of an image due to existence of a lens is reduced by appropriately setting the light directivity of a back light source, the distance between a liquid crystal layer and minute unit lenses, and the pitch of display units of the liquid crystal cell in the direction of the array of the minute unit lenses. The other method is disclosed in Japanese Laid-Open Patent Publication No. 7-120743, where reflection of external light from lenses is reduced by appropriately setting the height of the lenses, the pitch of the lenses, and the width of attached portions in the case that a liquid crystal display cell and lens convex portions are attached together via an adhesive or an adhesive layer.
The above conventional methods have the following problems. In Japanese Laid-Open Patent Publication No. 8-201796, it is disclosed that blurring of an image can be made unrecognizable by increasing the pitch of the display units in the column, reducing the distance between the liquid crystal layer and the minute unit lenses, and enhancing the light directivity of the back light source. However, no disclosure is made on the characteristics of the minute unit lenses. Therefore, when this technique is employed and the light directivity of the back light source is enhanced, if the minute unit lenses have distortion in the optical characteristic, such distortion in the viewing angle characteristic of the display is reflected, failing to obtain satisfactory display characteristics. For example, even if minute unit lenses having the most idealistic optical characteristics are successfully produced, the lenses must be attached to a liquid crystal display element via an adhesive layer. In this attachment, the contact state between the adhesive layer and the lenses is easily changed, and thus the minute unit lenses are inevitably distorted in the optical characteristics. The reason is that since the refractive index of the adhesive layer is roughly the same as that of the lenses, the tip portions of the minute unit lenses buried in the adhesive layer no more function normally as lenses. In this occasion, if the light directivity of the back light source is high, the distortion in the optical characteristics of the minute unit lenses influences the display characteristics of the liquid crystal display device.
According to the technique disclosed in Japanese Laid-Open Patent Publication No. 7-120743, no distortion will occur in the lens optical characteristics even when the light directivity of the back light source is high. However, in reality, in order to achieve distortion-free lens optical characteristics according to the technique disclosed in this literature, the ratio of the lens pitch to the width of attached portions must be 5 or more. It is very difficult to control the width of attached portions to attain this construction.
An object of the present invention is to provide a liquid crystal display device having a wide viewing angle and high display quality while using a light source with high directivity, and a lens sheet used for the liquid crystal display device.
The transmission type liquid crystal display device of the present invention includes: a liquid crystal display element including a pair of transparent insulating substrates placed to face each other with a gap therebetween, transparent electrodes being formed on inner surfaces of the substrates, and a liquid crystal material injected in the gap; a light source placed on the back side of the liquid crystal display element; and a light diverging element placed on the front side of the liquid crystal display element. The liquid crystal display device is designed so as to satisfy an expression   0.5  ≦                    ∫                              -            90                    ⁢          xc2x0                          90          ⁢          xc2x0                    ⁢              B        ⁢                  xe2x80x83                ⁢                  L          ⁡                      (            θ            )                          xc3x97        L        ⁢                  xe2x80x83                ⁢                  C          ⁡                      (            θ            )                          xc3x97                  Dif          ⁡                      (                                          10                ⁢                xc2x0                            ,              θ                        )                          ⁢                  ⅆ          θ                                    ∫                              -            90                    ⁢          xc2x0                          90          ⁢          xc2x0                    ⁢              B        ⁢                  xe2x80x83                ⁢                  L          ⁡                      (            θ            )                          xc3x97        L        ⁢                  xe2x80x83                ⁢                  C          ⁡                      (            θ            )                          xc3x97                  Dif          ⁡                      (                                          0                ⁢                xc2x0                            ,              θ                        )                          ⁢                  ⅆ          θ                    
where BL(xcex8) is a light emitting angle xcex8xe2x80x94luminance characteristic of the light source, that is, a luminance of the light source as a function of a light emitting angle xcex8, LC(xcex8) is an incident angle xcex8 to the liquid crystal display elementxe2x80x94transmittance characteristic of the liquid crystal display element in a bright state, that is, a transmittance of the liquid crystal display element as a function of an incident angle xcex8, and Dif(a, xcex8) is an incident angle xcex8 to the light diverging elementxe2x80x94transmittance characteristic of the light diverging element at a light receiving angle a, that is, a transmittance of the light diverging element as functions of a light receiving angle a and the incident angle xcex8.
The expression may be satisfied by adjusting the BL(xcex8) and the Dif(a, xcex8).
In a preferred embodiment, an adhesive layer is formed between the light diverging element and the liquid crystal display element, the light diverging element is a lens sheet including first unit lens portions with convex portions having a first height and second unit lens portions with convex portions having a second height smaller than the first height, the lens sheet is attached to the adhesive layer at the first unit lens portions.
In a preferred embodiment, the first unit lens portions of the lens sheet are attached to the adhesive layer, while the second unit lens portions are away from the adhesive layer, and the lens sheet has an optical characteristic of Dif(a, xcex8)=nxc3x97g(a, xcex8)+(1xe2x88x92n)xc3x97f(a, xcex8) where g(a, xcex8) is an optical characteristic of the first unit lens portions, n is a proportion in area of the first unit lens portions to an entire lens sheet, f(a, xcex8) is an optical characteristic of the second unit lens portions, and 1xe2x88x92n is a proportion in area of the second unit lens portions to the entire lens sheet.
In a preferred embodiment, the light source has directivity that a light emitting angle at which a luminance is a half of the luminance in a direction normal to a front surface of the liquid crystal display element is 15xc2x0 or less.
In a preferred embodiment, the light source has directivity that a light emitting angle at which a luminance is a half of the luminance in the direction normal to the front surface of the liquid crystal display element is 5xc2x0 or less, and the light diverging element has an optical characteristic that Dif(0xc2x0, 10xc2x0)/Dif(0xc2x0, 0xc2x0) is 0.4 or more.
A transmission type liquid crystal display device of the present invention includes: a liquid crystal display element including a pair of transparent insulating substrates placed to face each other with a gap therebetween, transparent electrodes being formed on inner surfaces of the substrates, and a liquid crystal material injected in the gap; a light source placed on a back side of the liquid crystal display element; and a lens sheet placed on a front side of the liquid crystal display element via an adhesive layer. The lens sheet includes: first unit lens portions with convex portions having a first height, and second unit lens portions with convex portions having a second height smaller than the first height, wherein both of the convex portions having the first height and the convex portions having the second height face the adhesive layer, the first unit lens portions are attached to the adhesive layer, while the second unit are away from the adhesive layer.
In a preferred embodiment, a ratio of the second height to the first height is 3/5 and a proportion in area of the first unit lens portions to an entire lens sheet is about 0.2.
In a preferred embodiment, the light source has directivity that a light emitting angle at which a luminance is a half of the luminance in a direction normal to a front surface of the liquid crystal display element is 15xc2x0 or less.
In a preferred embodiment, the light source has directivity that a light emitting angle at which a luminance is a half of the luminance in the direction normal to the front surface of the liquid crystal display element is 5xc2x0 or less, and the lens sheet has an optical characteristic that Dif(0xc2x0, 10xc2x0)/Dif(0xc2x0, 0xc2x0) is 0.4 or more when Dif(a, xcex8) is an incident angle xcex8 to the lens sheetxe2x80x94transmittance characteristic of the lens sheet at a light receiving angle a.
A lens sheet of the present invention is attached to a front side of a liquid crystal display element via an adhesive layer. The lens sheet includes: first unit lens portions with convex portions having a first height, and second unit lens portions with convex portions having a second height smaller than the first height, wherein both of the convex portions having the first height and the convex portions having the second height face the adhesive layer.
The lens sheet preferably has an optical characteristic that Dif(0xc2x0, 10xc2x0)/Dif(0xc2x0, 0xc2x0) is 0.4 or more when Dif(a, xcex8) is an incident angle xcex8 to the lens sheetxe2x80x94transmittance characteristic of the lens sheet at a light receiving angle a.
In a preferred embodiment, a ratio of the second height to the first height is 3/5 and a proportion in area of the first unit lens portions to an entire lens sheet is about 0.2.